Tightened border security has led Mexican drug cartels and human smugglers to use more sophisticated routes to enter the U.S. and this week Border Patrol agents discovered yet another underground tunnel loaded with 15,000 pounds of U.S.-bound drugs.

The tunnel, connecting east San Diego California to the Mexican town of Tecate, is one of dozens discovered in the last few years crossing the U.S.-Mexico border. The 15,000-pound stash of marijuana was making its way north when a Border Patrol agent working with a specially trained dog discovered the tunnel during a routine patrol shift.

Like many of the previously discovered tunnels, federal authorities say this one is sophisticated and reinforced with high-quality construction materials and a ventilation system. Violent drug cartels and human smugglers don’t cut corners building their precious underground passageways because the lucrative profits from their illegal businesses make the investment well worth it.

In the last six years more than 40 tunnels, connecting Mexico to the U.S., have been discovered and most have led into Nogales Arizona and San Diego California. In 2006, U.S. authorities found two tons of marijuana in a massive tunnel that connected Tijuana to San Diego.

Spanning the length of about eight football fields, the elaborate underground structure had a sophisticated lighting and ventilation system as well as a ground-water pumping system and an efficient pulley operation to easily move large amounts of drugs.

U.S. authorities say that heightened border security since the 2001 terrorist attacks have led to a huge increase in U.S. to Mexico tunneling. In fact, Immigration and Customs Enforcement (ICE) actually created a special tunnel task force that works closely with the Drug Enforcement Administration and Border Patrol to proactively look for tunnels.